Vector saliva appears to play a critical role in the successful bloodfeeding of hematophagous arthropods. Components of saliva serve to enhance pathogen transmission and host infection. Recent studies on the pharmacological properties of the saliva from Ixodes scapularis, the principal vector of Lyme disease spirochetes in the eastern U.S., reveal that its saliva contains substances that enhance bloodfeeding and perhaps determines vector competence. In particular, anticomplement activity in this tick's saliva may confer protection against complement-mediated killing of Borrelia burgdorferi and host rejection of the tick. In addition, saliva components modify protein expression in B. burgdorferi, perhaps enhancing infectivity of this bacterium, and certainly affecting the host response to infection. The proposed research will: (I) identify and characterize components of I. scapularis saliva responsible for anticomplement activity, and that affect protein expression in B. burgdorferi; (2) assess the protective effect of tick saliva fractions collected from various ixodid ticks (e.g. I. scapularis, Dermacentor variabilis, Amhlyomma americanum) on B. burgdorferi in a complement-based borreliacidal assay; (3) determine the effect of tick saliva on protein expression in B. burgdorferi and on the host immune response to infection; (4) evaluate the pathogenicity and invasiveness of B. burgdorferi incubated in tick saliva and tick gut extract; and (5) assess the potential of using tick salivary proteins, such as the I. scapularis anticomplement protein (Isac), as immunogens to induce protective humoral and cellular immunity to tick feeding and B. burgdorferi transmission. Attempts will be made to clone those genes encoding both specific tick salivary proteins or peptides and saliva-induced B. burgdorferi proteins. These will then be expressed in a suitable plasmid vector. Purified recombinant proteins will be used in various vaccination protocols in an attempt to neutralize the tick anticomplement activity in vitro and in vivo, and to protect the host against Borrelia infection. Thus, one long range goal of this research is to evaluate a vaccine strategy active against the tick vector prior to pathogen transmission and the transmission process itself. An additional goal is to evaluate new B. burgdorferi proteins as potential vaccine candidates that specifically target the pathogen upon entry into the host. Finally, the saponin adjuvant QS-21 will be evaluated as an stimulant of humoral and cellular immunity.